[Update #2]: The day has finally come - MTG Arena has now made its way onto Steam!
[Update]: Magic: The Gathering is extending its Standard rotation from two to three years in order to help the floundering format on tabletop.
After teasing it for what feels like forever, Wizards of the Coast has now finally decided to settle on a date for MTG Arena's big Steam release. So if you're in the mood for a modern, digital adaptation of Magic: The Gathering, make sure to mark May 23rd on your calendar... or just give the standalone version a try right now.
Much like the original, the Steam version of MTG Arena will be free-to-play and give you access to all of the game modes from the very beginning. If you're only just starting out it'll most likely take you a couple of months to build up a somewhat competitive collection, but once that's done keeping up with new sets is generally fairly easy, especially if you're a fan of the draft format.
Regardless of which version you decide to go for, everything is going to be handled via your MTG Arena account. In other words, there's going to be full cross-play between the various versions (both PC and mobile), and your collection will follow you to whichever platform you decide to play on.
The one thing I'm not sure about is whether the Steam version will come with the usual knickknacks like achievements and trading cards. Personally, I hope so because the idea of a trading card game rewarding you with another, smaller batch of trading cards is kind of funny to me.
Whatever the case may be, I would highly recommend giving MTG Arena a try if you're curious about Magic: The Gathering or card games in general. I've been playing it on and off for the past couple of years, and despite the relatively slow pace of quality-of-life improvements, I've only really got positive experiences with it.
For now, however, you can learn more about MTG Arena, as well as follow its upcoming Steam release, over at (you guessed it!) Steam. Enjoy, and I'll make sure to let you know once it actually arrives.